Hi,
In response to your question, the Tamron 150-600mm is your very best route to go.
Using the multiplier will only cause you to miss out on fast shots as you are working through 2 sets of readings, rather than just one with a single lens.
The G2 is supposed to be a lot better than the first one, as the first one is what I use with my Nikon D4S, but working with multipliers and a large lens, it will really be frustrating for you.
My first choice would be, get the Tamron 150-600mm G2 instead of a multiplier or a Prime lens.
I have also owned and worked with the Nikor Prime 300 with a multiplier and it really did not improve image quality nor speed and I was eventually out of a lot of money because the multiplier eventually messed up the auto-focus on the lens, making it worthless to work with. So, don't risk your expensive equipment with multipliers. It is just a bad investment in the long run.
Definitely spend your money on the Tam G2. You will be so much happier with it and you will catch far more with it being versatile in the focal lengths. With Primes, you are always locked in and it gets to be a real pain when you have to physically move to adjust for distance out in the wild. That is never fun. Besides the fact that the Primes weigh a ton to haul around.
Hope this helps and my next long lens I will be getting is the Tam G2, myself. I use my first one all the time, but just came into a full Canon outfit and the G2 is next on my list.
You have SO many options with the G2 that you cannot get with multipliers or Primes.
Have fun!
I shot this image with my Tamron 150-600mm as we have a mating pair of Eagles here at my house. They have been here since 2014, when I first spotted them. I use my Tam 150-600 more than I do any of my other lenses. Just love it.
